Improved API for Designator

[marten]

It would be better if Designator would communicate more clearly to Panoptes about why it's returning an empty array. Currently that could be because the workflow wasn't loaded yet, but it can also happen if it is loaded but there is nothing to select.

Currently the response is just

[1,2,3,4]

Proposed response is:

{ 
  "status": "selected_subjects" // or "cache_populating", "nothing_to_select_for_user", maybe more?
  "subject_ids": [1,2,3,4]
}

We may not be interested in changing Panoptes to actually do anything different with this additional information at the moment, but at the very least it'll be helpful in debugging if we ever request subjects by hand. Going from a top-level array response to a top-level hash response is also a prerequisite for any other thing we might want to return.

[camallen]

I'm all for this, might be a nice piece of work to pair with zach and/or amy on?